Increasing bandwidth of communications systems has led to the introduction of new modulation formats such as four-level pulse amplitude modulation (PAM-4), and the introduction of forward error correction (FEC)-protected links into new communications standards such as Institute of Electrical and Electronics Engineers (IEEE) 802.3bs and 802.3cd.
Whether signals are communicated over electrical or optical links having FEC protection, error propagation conditions can occur at certain tap values for decision feedback equalizer (DFE) circuitry in the receiver chain of the communications systems. The error propagation conditions can be relatively severe for PAM-4 signals when compared with non-return-to-zero (NRZ) binary signals. For example, equivalent DFE tap weights that are greater than 0.5 can cause the error distribution statistics to change from a random mode to a burst mode.
One possible solution for mitigating or preventing burst mode errors includes controlling an error limit of an input signal (i.e., before applying FEC) to be less than a predefined error limit. Another possible solution includes a single DFE tap into an equalizer chain at the optical receiver. In some cases, the DFE tap may be bypassed and/or may be limited to a certain maximum value, e.g., using appropriate firmware control settings. While these solutions can be effective to mitigate or prevent burst mode errors, the performance of the communications system may be limited unnecessarily.
Because the maximum value of the DFE tap will depend on the error limit of the input signal, the error propagation condition can depend on the actual channel configuration. Additionally, channel variations can occur over time due to environmental conditions and/or aging, but this is generally less influential than the channel configuration.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.